Abstract
We have explored two areas of approximately rigorous calculations for computing nonempirical wave functions for heavy and/or large molecules orders of magnitude faster than with conventional abβinitio methods but with the same chemical accuracy.
First, we have developed and used a series of programs (starting from our new fast sets of abβinitio Gaussian SCF and SCFβCI programs) incorporating abβinitio effective core model potentials (MODβPOT) which allow one to treat only the valence electrons explicitly, plus a charge conserving integral prescreening, which cuts down significantly on the number of integrals that have to be calculated, stored, or processed for a large molecule. We have named this latter procedure VRDDO (variable retention of diatomic differential overlap). With these MODPOT and MODPOT/VRDDO methods we have explored a variety of small, medium, and large systems ranging from electron affinities of atoms through to molecules of biological interest and large boron hydrides. The results compared to abβinitio SCF or SCF/CI calcuations are very good, usually within 0.001 to 0.002 a.u. for orbital energies and gross atomic populations (GAPS) and even better along potential energy curves.
Secondly, we have explored the use of the MSβ__X__Ξ± method for less conventional molecules and properties than those for which it is customarily employed.